I frequent a few, but post mostly on this one. AHB is my favourite but the others fill in the gaps when things are slow here. But I think for a sense of community and a forum where no one is afraid to make a post, AHB is the best.
I regularly read and post on the Brews and Views forum of HBD:
http://hbd.org/cgi-bin/discus/discus.cgi
Also the rec.crafts.brewing Google forum, mainly for the HBD daily digest that I can't seem to read if I go directly to the HBD, not sure why:
http://www.brewing-forum.net/brewing/
oz.craftbrewers.org I get the daily digest sent to my email account, post on occassion.
Also do a swing past the Northern Brewer and More Beer forums, plus more recently I've browsed over the Brewboard and Brewcommune.
I post a little bit on Grumpys but I get frustrated on that forum for the most part and don't have a lot of time for it. No offense intended to anyone on it or those who read it but I don't tend to find much info of interest or value on that forum, but in my opinion it serves a slightly different readership and that's perfectly fine. They could really do with a registered membership thing to cut down on trolls though as stated above.
So yep. Read a few different forums and pick up bits of info of all of them, I like to read about brewing.
Cheers, Justin